Solar cells are generally made of semiconductor materials, such as silicon (Si), which convert sunlight into useful electrical energy. A solar cell contact is typically made of thin wafers of Si in which the required PN junction is formed by diffusing phosphorus (P) from a suitable phosphorus source into a P-type Si wafer. The side of the silicon wafer on which sunlight is incident is usually coated with an anti-reflective coating (ARC) to prevent reflective loss of sunlight. This ARC increases the solar cell efficiency. A two dimensional electrode grid pattern known as a front contact makes a connection to the n-side of silicon, and a coating of predominantly aluminum (Al) makes connection to the p-side of the silicon (back contact). Further, contacts known as silver rear contacts, made out of silver or silver-aluminum paste are printed and fired on the p-side of silicon to enable soldering of tabs that electrically connect one cell to the next in a solar cell module. These contacts are the electrical outlets from the PN junction to the outside load.
The solar cell design in widespread use today has a PN junction formed near the front surface, where sunlight is received, which creates an electron flow as light energy is absorbed into the cell. The conventional cell design has one set of electrical contacts on the front side of the cell, and a second set of electrical contacts on the back side of the solar cell. In a typical photovoltaic module these individual solar cells are interconnected electrically in series to increase the voltage. This interconnection is typically accomplished by soldering a conductive ribbon from the front side of one solar cell to the back side of an adjacent solar cell.
Back-contact silicon solar cells have several advantages compared to conventional silicon solar cells. One is that back-contact cells have a higher conversion efficiency due to reduced or eliminated contact obscuration losses (sunlight reflected from contact grid is unavailable to be converted into electricity). Another is that assembly of back-contact cells into electrical circuits is easier, and therefore cheaper, because both conductivity type contacts are on the same surface. As an example, significant cost savings compared to present photovoltaic module assemblies can be achieved with back-contact cells by encapsulating the photovoltaic module and the solar cell electrical circuit in a single step. Yet another advantage of a back-contact cell is better aesthetics through a more uniform appearance. Aesthetics is important for some applications, such as building-integrated photovoltaic systems and photovoltaic sunroofs for automobiles.
FIG. 1 illustrates a generic back-contact cell structure as known in the art. The silicon substrate may be n-type or p-type. One of the heavily doped emitters (n++ or p++) may be omitted in some designs. Alternatively, the heavily doped emitters could be in direct contact with one another on the rear surface. Rear-surface passivation helps to reduce the loss of photogenerated carriers at the rear surface, and helps reduce electrical losses due to shunt currents at undoped surfaces between the contacts.